1. Field of the Invention
This invention relates generally to systems for locating and tracing buried objects and more particularly to a system for detecting and tracing buried objects by means of automated active and passive signal event detection.
2. Description of the Related Art
There are many situations where is it desirable to locate buried utilities such as pipes and cables. For example, before starting any new construction that involves excavation, worker safety and project economic concerns require the location and identification of existing underground utilities such as underground power lines, gas lines, phone lines, fiber optic cable conduits, CATV cables, sprinkler control wiring, water pipes, sewer pipes, etc., collectively and individually herein denominated “buried objects.” As used herein, the term “buried objects” includes objects located inside walls, between floors in multi-story buildings or cast into concrete slabs, for example, as well as objects disposed below the surface of the ground. If excavation equipment such as a backhoe hits a high voltage line or a gas line, serious injury and property damage may result. Unintended severing of water mains and sewer lines generally leads to messy and expensive cleanup efforts. The unintended destruction of power and data cables may seriously disrupt the comfort and convenience of residents and bring huge financial costs to business.
Accordingly, the art is replete with proposed solutions to the buried object locating problem. For example, it is known to locate buried ferromagnetic objects either by detecting a localized change in free-space permeability with an inductive circuit element or by using a magnetic sensor to detect the fixed magnetic field (internal magnetic moment) emanating from the object. The presence of such ferromagnetic objects also distorts the Earth's magnetic field in a manner that is known to have utility for locating. Still other buried objects, such as conductive lines and pipes, may be located by first applying an external electromagnetic signal to the object to thereby energize the object with a nonzero frequency magnetic field that may be detected by a magnetic sensor. For example, an external electrical signal source (transmitter) having a frequency in the range of approximately 4 Hz to 500 kHz has a well-known utility for energizing conductive objects by direct electrical coupling to permit their location. Some buried cables, such as power lines and some communication lines, for example, are already energized and therefore characterized by the emission of an electromagnetic signal that includes a non-zero frequency magnetic field that may be detected by a magnetic sensor. These examples of active and passive location of buried long conductors are also commonly denominated “line tracing.” As another example, an external transmitter, beacon or duct probe (a “sonde”) is an external electromagnetic signal source having well-known utility for marking the location of any non-conductive buried object into which it may be physically introduced. A sonde typically includes a coil of wire wrapped around a ferromagnetic core that is packaged for insertion into a buried nonconductive conduit, such as a plastic utility runway or a concrete water pipe.
When locating buried objects before excavation, it is further desirable to determine the approximate depth of the objects. This is generally attempted by measuring the characterizing emission field strength at two locations and analyzing the differences to infer the location of the emission source. However, there are many instances where the land that is to be excavated may be traversed or crisscrossed by several different utilities such as an electrical power cable, a water line, a gas line, a sewer pipe and a communications line. It is highly desirable to be able to determine their paths and their depths all at the same time. Also, many sites are host to a variety of overhead power and related lines, which emit electromagnetic fields that cannot be readily segregated from the emissions of similar buried lines. Some transmitters known in the art can produce several different signals at different frequencies for application to the same underground object or even to different underground objects, but a problem with these systems arises when several pipes are located in the same area and the location of all pipes is desired. Signals transmitted by several pipes can interfere and complicate the detection process.
Practitioners in the art have proposed numerous refinements to the magnetic field detector intended to facilitate the location of underground objects. For example, in U.S. Pat. No. 4,843,324, Humphrey, Jr. et al. teach that band-limited measurements from a single sensor in two positions are more accurate than two simultaneous measurements from two spaced-apart sensors for determining the magnetic gradient required to estimate buried object depth. U.S. Pat. No. 6,777,923 issued to Pearson discloses a method for detecting the magnetic field gradient by means of two magnetic sensors separated by a fixed distance. Pearson teaches the use of a simple threshold detection method that permits the dual sensors to operate in a self-calibrating mode to ensure accuracy of the measured gradient. Pearson tests the measured gradient against a predetermined threshold to produce an indicator signal representing a simple “object present” event, which is communicated to the user by means of a User Interface (UI). Similarly, U.S. Pat. No. 6,107,801 issued to Hopwood et al. discloses a method for estimating the distance from sensors to sonde dipole field by comparing the ratio of two orthogonal magnetic field basis elements to that computed for a sonde field predicted in accordance with a sonde a tilt detector output. In U.S. Pat. No. 6,268,731, Hopwood et al. disclose a magnetic field detector having three spaced-apart sensor arrays each having differing sensor orientations. Hopwood et al. suggest that the user move along while sweeping their locator from side to side and listening to audio signals representing the relative values of the three sensor array outputs, thereby assigning the entire location event detection burden to the user. In U.S. Pat. No. 6,140,819, Peterman et al discloses an apparatus with two spaced-apart sensors that employs a phase-lock loop (PLL) circuit for locking a magnetic field sensing circuit to the signal used to energize a buried utility line, thereby permitting the continuous indication of a magnetic field gradient representing the estimated object depth (instead of the earlier “halt-and-read” depth estimation method). None of these useful disclosures suggests a dispositive solution to the well-known magnetic gradient accuracy problems encountered with one-dimensional (1D) and two-dimensional (2D) sensor arrays, which may be understood by recalling that a magnetic field gradient in three-space has nine components, five of which are independent. These five independent components cannot be completely resolved with less than five independent magnetic sensor measurements.
Modern utility line tracing places complex demands on the locator user, who may be obliged to detect one or more buried objects in a crowded or noisy environment. Presently in the art, the actual location and tracing of a buried object obliges the locator user to receive, review and evaluate a variety of signals from the magnetic signal detection apparatus UI. For example, it is known to provide an audio UI signal representing the detection of a magnetic field that is stronger than a predetermined threshold. As the locator user hears the audio UI signals and moves the locator sensor about, the user must evaluate how the signal changes with respect to the sensor motion and adapt the motion to this information in order to eventually conclude a probable location for the buried object characterized by the emission of the magnetic field being detected. This process may be described as a series of “event” detections by the user; that is, for example, the user first detects an event representing a nearby pass of the sensor over the object sought, whereupon the user changes the sensor positions in a manner intended to elicit another event and so forth until a series of such events informs the user of the object location. While this may appear simple in principle, the reality is that the user must recursively process a large amount of information to determine an object location even when such determination is possible.
Effective detection and tracing of utility lines is vital to the safety of field personnel for many reasons; for example, the unplanned rupture of a high-pressure natural gas line can endanger the lives of everybody in the vicinity. Because of the important physical safety issues involved in line detection and tracing, both for the excavators and the locator users preceding them, several practitioners have proposed improving the quality of utility line location and tracing by moving some of the event-detection burden from the locator user to the locator apparatus itself. Such a system must provide for the simultaneous detection and identification of either a passively-emitting buried object such as a ferromagnetic mass or an energized power cable or an actively-energized buried object such as a conductive pipe energized by means of an external transmitter signal or a non-conductive conduit occupied by an energized sonde, or all simultaneously, for example.
Many practitioners propose various solutions to this multiple event-detection and separation problem. For example, U.S. Pat. No. 5,093,622 issued to Balkman discloses a locating apparatus that provides a new formula for comparing the output signals from two spaced-apart magnetic sensors to automatically detect a “cross-over” event when the sensor passes directly over the buried object sought. Similarly, U.S. Pat. No. 5,621,325 issued to Draper et al. discloses a method for using two orthogonal magnetic field basis vectors to discriminate between actual detections and “ghost” detections by signaling the actual event detection to the user while ignoring the “ghost” detections. However, these useful suggestions alone do little to shift the increased event processing burden from the user to the apparatus.
The above commonly-assigned patent applications propose several improvements to the magnetic field measurement and line locating art, including the use of simultaneous measurement of magnetic field vectors in a plurality of independent frequency regions and the introduction of multiple 3D sensor arrays for measuring magnetic field vectors and the introduction of an improved Graphical UI for line tracing.
The measurement of the magnetic field gradients can be more useful than the simpler measurement of the magnetic field vectors for detecting electromagnetic emissions because magnetic gradiometry is less susceptible to distant interfering signals such as those from overhead power lines. For example, in a dipole field, the gradient declines with the fourth power of distance from an emitter while the field declines with the third power of distance. In a cylindrical field, the gradient retains this noise-reduction advantage by declining as the square of the distance from the emitter while the field declines linearly with distance. Although magnetic gradiometry is known in the art, true gradient measurement requires at least five independent simultaneous magnetic field measurements to completely determine the nine magnetic gradient tensor elements. To reduce the expense of the numerous independent sensors required, magnetic gradients are often merely approximated in the art by employing assumptions about the magnetic field geometry and by ignoring or resolving in some other manner the resulting location ambiguities. This well-known difficulty may be appreciated with reference to, for example, a Department of Justice Research Report by Czipott [Czipott, “Stand-Off Detection and Tracking of Concealed Weapons Using Magnetic Tensor Tracking,” U.S. Department of Justice Research report #189583, 08 Aug. 2002], who discusses the importance of measuring the entire magnetic gradient tensor in facilitating the location of ferromagnetic objects. Also, U.S. Pat. No. 5,777,477 issued to Wynn describes the practical difficulties in measuring the five independent gradients of a magnetic field at a single point while in motion, suggesting that even when measuring the 3D magnetic vector to eliminate “ghost” solutions to the location vector arising from the gradient tensor alone, stationary measurement is usually necessary for detecting weak ferromagnetic signatures. Wynn discloses a computational method that permits continuous nonambiguous solutions for the location vector when both the entire five-element gradient tensor and the complete three-element magnetic field vector are measured at each point.
Another well-known portable locator problem is the convolution of any locator system user motion with the received emission signal. In operation, a portable locator system is typically swung side to side or about in a large arc to obtain an initial estimate of the direction to a buried object such as a utility line. Even when tracing a buried utility line following its detection, small path deviations may cause signal phase reversals at the sensors whose axes are aligned substantially perpendicular to the local magnetic field vector. This problem arises in sensor coil embodiments where sensor signal sensitivity is generally proportional to the sine of the angle between the sensor coil axis and the local field vector. When the sensor coil axis is substantially aligned with the local field, the near-zero value for the sine of that angle fluctuates rapidly with the angle so that minute angular changes arising from locator system motion can cause large changes in sensor signal amplitude and phase. Until now, there were no proposed solutions to this locator sensor alignment problem.
Accordingly, there is still a clearly-felt need in the art for an improved method for automatic location of buried objects in a crowded and noisy environment. These unresolved problems and deficiencies are clearly felt in the art and are solved by this invention in the manner described below.